在Linux驱动程序中轮询系统调用

Question

I am learning Linux internals. So I came across the poll system call. As far as I understand, it is used by drivers to provide notification when some data is ready to be read from device and when we have data ready to device.

If device do not have any data to read, process will get sleep and wake up when data become available and vice versa for write case.

Can someone provide me concrete understanding of poll system call with some real example?

Answer 1

poll and select (the latter is very similar to poll with these differences) sys calls are used in so called asynchronous event-driven approach for handling client's requests.

Basically, in network programming there are two major strategies for handling many connections from network clients by the server:

1) more traditional: threaded or process-oriented approach. In this situation network server has main proccess which listens on one specific network port (port 80 in case of web servers) for incomming connections and when connection arrives, it spawns new thread/process to handle this new connection. Apache HTTP server took this approch.

2) aforementioned asynchronous event-driven approach where (in simplest case) network server (for example web server) is application with only one process and it accepts connections (creating socket for each new client) and then it monitors those sockets with poll/select for incoming data. Nginx http web server took this approch.